1. Field of the Invention
The present invention relates to x-ray diagnostic systems having a control loop or automatic control system, and in particular to such diagnostic systems having a radiation detector for detecting the mean radiation dose and a means for influencing or regulating at least one radiographic exposure value in dependence upon the output of the detector.
2. Description of the Prior Art
X-ray diagnostic systems are known having a control loop which includes an integrator for integrating the signal from a radiation detector and for automatically disconnecting the high voltage supply to the x-ray tube when the integrated output of the detector reaches a predetermined dose per image. A control loop of this type is generally referred to as an automatic exposure timer. In x-ray diagnostic systems of this type, optimally exposed x-ray images are obtained. Another type of automatic exposure timer is known which utilizes a photomultiplier connected after the x-ray image intensifier as the radiation detector, the photomultiplier detecting the mean image brightness in the ray trace between the output fluorescent screen of the image intensifier and an image recording device (such as a television camera) in a predetermined region. False exposures may nonetheless occur when the contrast in the region detected by the radiation detector varies during a series. Such false exposures may arise, for example, when an x-ray contrast agent flows into the detected region because the presence of the contrast agent causes the mean image brightness to be maintained substantially constant, due to the high radiation absorption of the contrast agent, and this artificial image brightness may deviate considerably from a diagnostically significant value.